In recent years, a considerable amount of research has been conducted in an effort to develop a low profile (thin), full color, electronic display system which does not rely upon conventional cathode ray tube technology. In systems such as television receivers, computer monitors, avionic displays, aerospace displays, and other military-related displays, the elimination of cathode ray tube technology is desirable for several reasons, which reasons will be detailed in the following paragraphs.
More particularly, cathode ray tubes are typically characterized by extremely large depth dimensions and thus occupy a considerable amount of floor or counter space. As a matter of fact, the depth dimension may equal the length and width dimensions of the viewing screen. Also, because cathode ray tubes require an elongated neck portion to provide for the acceleration of an electron beam from the electron gun to the faceplate of the cathode ray tube, they are quite irregular in shape. Additionally, since cathode ray tubes are fabricated from relatively thick glass, they are inordinately heavy, extremely fragile and readily breakable. Finally, cathode ray tubes require a relatively high voltage power supply in order to sufficiently accelerate the electron beam and thus sustain the displayed image.
The reader can readily appreciate the fact that all of the foregoing problems experienced with or shortcomings of cathode ray tubes are exacerbated as the size of the viewing screen increases. Since the current trend, and in fact consumer demand, is toward larger screens; weight, breakability, placement, etc. represent significant commercial considerations. Accordingly, it should be apparent that cathode ray tubes are and will continue to be inappropriate for those applications in which weight, fragility and portability are important factors.
One system which can eliminate all of the aforementioned shortcomings of the present day cathode ray tube is the flat panel liquid crystal display in which a matrix array of liquid crystal picture elements or pixels are arranged in a plurality of rows and columns. Liquid crystal displays may typically be either reflective or transmissive. A reflective display is one which depends upon ambient light conditions in order to be viewed, i.e., light from the surrounding environment incident upon the side of the display facing the viewer is reflected back to the viewer. Differences in the orientation of the liquid crystal material within each liquid crystal pixel cause those pixels to appear either darkened or transparent. In this manner, a pattern of information is defined by the two dimensional matrix array of darkened (or transparent) pixels. However, and as should by now be apparent, reflective liquid crystal displays cannot be used in a dark or low light environment since there is no light available for reflection off the viewing surface of the display.
Conversely, transmissive liquid crystal displays require the use of illuminating means such as a lamp array operatively disposed on the side of the matrix array of picture elements opposite the viewer. This illumination means or backlight may further include a backreflector adapted to efficiently redirect any stray illumination towards the matrix array of rows and columns of picture elements, thus ensuring that the displayed image is as bright as possible (given the lighting capabilities and characteristics of the backlighting scheme being employed). The instant invention is specifically directed to the field of backlit, high resolution liquid crystal electronic displays.
The characteristics of the backlighting scheme are very important to both the quality of the image displayed by the matrix array of picture elements of the liquid crystal display and the profile, i.e., the thickness dimension, of that liquid crystal display. Accordingly, a great deal of the aforementioned research in the field of said flat panel electronic displays has been dedicated to the design and fabrication of backlighting systems which optimize certain viewing and structural parameters of those flat panel displays. Characteristics which are acknowledged by experts as the most important in the design of optimized backlighting assemblies include: 1) uniformity across the large surface areas illuminated by the light provided by the backlight, i.e., the intensity of the light must be substantially the same at each pixel of the large area liquid crystal display; 2) high brightness illumination provided by the backlight thus yielding a sharp, readily readable image to a remotely positioned viewing audience; 3) a low profile so that a flat panel liquid crystal display is substantially flat and can be operatively disposed for viewing without occupying an undue amount of available space; 4) the overall design of the backlight which takes into consideration the number, configuration, and redundancy of lamps; 5) the heat effect caused by the number, configuration, redundancy and type of the lamps; and 6) the total power consumed by the lighting scheme which represents an extremely important consideration in hand held (portable) television units.
A number of different backlight configurations, all of which included a plurality of discrete optical components disposed between the plane of the source of backlit radiation and the plane of the matrix array of liquid crystal pixels, have been designed in an effort to maximize each of the desirable characteristics recited hereinabove. For example, those of ordinary skill in the art of liquid crystal display backlighting have attempted to use light diffusers in an effort to achieve a more uniform distribution of projected light across the entire viewing surface of the liquid crystal display. This technique, while useful for improving the uniformity of projected light, deleteriously affected the intensity of that projected light resulting in light appearing soft or washed-out. Thus, additional lamps were required when such light diffusers were employed, resulting in an increased heating effect upon the display. Further, due to the fact that such light diffusers were, of necessity, positioned an operative distance from both the source of backlighting as well as from the matrix array of liquid crystal pixels, the depth dimension or profile of the electronic, flat panel display was significantly increased.
A second technique employed to enhance the quality of the backlight (and hence the quality of the displayed image) is to operatively dispose a light collimating lens, such as a fresnel lens, between the source of the backlight and the matrix array of liquid crystal picture elements. This design expedient has the effect of producing an intense, sharp image from a minimal number of lamps, while simultaneously providing a high degree of uniformity of projected radiation across the entire viewing surface of even large area displays. However, due to the nature of collimated light, the viewing angle of a display equipped with such a light collimating lens is limited. Indeed, viewing of the displayed image is impossible from any angle other than directly straight-on. Accordingly, a backlit display which employs only a light collimator without a mechanism for increasing the viewing angle has limited commercial applicability, and is wholly inappropriate for the gigantic markets related to television and computer monitors. Additionally, collimating means, such as fresnel lenses, are characterized by an operative focal length. (The focal length is that distance from the light source at which said lens must be disposed in order to properly collimate light emanating from said light source.) Thus, the light collimator has the undesirable effect o increasing the profile of the liquid crystal display. Also, backreflectors are inappropriate for use with light collimating. This is because light reflected from the backreflector does not originate from a position which is at the focal length of the collimating lens. Hence, light reflected from said backreflector will not be collimated. This results in localized bright spots on the surface of large area displays, degrading the quality of the displayed image.
In a effort to achieve the advantages of both light collimation and light diffusion, routineers in the backlit, flat panel liquid crystal display art have attempted to incorporate both a discrete light diffuser and a discrete light collimator into the same backlit liquid crystal display. Optically speaking, the results have been satisfactory only to the extent that the quality of the displayed image is relatively sharp, intense and uniform; while said image is visible over a relatively wide viewing angle. However, in order to maximize the optical effect of utilizing the diffuser-collimator combination, it was necessary to operatively space the collimator from the source of backlighting radiation, and then to space the diffuser from both the plane of the collimator and the plane of the matrix array of liquid crystal pixels. The result was a substantial increase in the profile, i.e., the depth dimension of the liquid crystal display. Indeed, in typical liquid low profile crystal display systems which include both a light collimator and a light diffuser, the distance from the light source to the diffuser is approximately 17 millimeters. It can thus be seen that by including both diffusing and collimating optical components, the profile of a typical flat panel liquid crystal display is significantly increased, thus eliminating one of the principle advantages of liquid crystal display systems; i.e., compactness. One method of reducing the depth profile and providing the foredescribed improved optical effect is disclosed in copending United States patent application No. 473,039, filed Jan. 31, 1990, assigned to the assignees of the instant application, the disclosure of which is incorporated herein by reference.
While the commonly assigned and copending application improved the profile and optical characteristics of prior art electronic displays having a given figure of merit for intensity of illumination per unit area, that application did not deal with improvements in the lighting efficiencies so as to reduce the number of lamps, the thermal effects of those lamps and the power consumption of those lamps. Accordingly, there still exists a need in the flat panel liquid crystal display art for an improved lighting/optical arrangement which provides a bright, uniform image of high contrast and capable of being viewed over a wide viewing angle, while maintaining a narrow profile and minimizing power consumption and thermal inconveniences.